Young infants' preferences to listen longer to infant-directed speech (IDS) over adult-directed speech (ADS) has been extensively studied (Fernald, 1985; Cooper, 1997; Cooper & Aslin, 1990; Werker & McLeod, 1989). One widely accepted outcome of this research is that speech preferences are based on prosodic characteristics of IDS, specifically the highly modulated voice pitch or fundamental frequency (F0) (Fernald & Kuhl, 1987). However, the focus on young infants has left the speech preferences of older infants largely ignored, especially during the transition to expressive language use, a time when infants are rapidly developing sensitivity to a wide range of linguistic aspects of the speech signal. Another result of the focus on young infants is that the role of non-prosodic aspects of IDS has not been explored. The hypothesis that underlies this proposal is that, beginning between 4 - 6 months of age, infants' speech preferences shift from a prosodic basis to a reflection of their developing speech perception capabilities, especially attention to and memory for words or "word-like" phonetic strings. It is proposed that one significant factor in determining infants' speech preferences during this period is the frequent repetition of words, phrases and utterances in IDS. Thus the main goals of the proposed research are: 1) to extend our knowledge and understanding of speech preferences from early infancy (e.g., 3-4 months) through the transition to expressive language use (e.g., 16-18 months); 2) to understand the role of non-prosodic factors, specifically repetition, in IDS preferences during this period; 3) to determine whether the kind of repetition infants hear in speech from adults reflects developmental changes in their preferences. To accomplish these goals, two avenues of study are proposed: 1) an integrated series of experiments will test the hypothesis that the development of speech preferences reflects infants' increasing ability to attend to and remember constituent linguistic structure (e.g., words, phrases & utterances) in the speech signal and phrases in instrumental music; 2) an observational study of mother-infant (M-I) interactions to explore the relationship between repetition in IDS and infant perceptual preferences.